After all, you and I are built from a long chain of chemicals, half from our moms, half from our dads—the famous 3.2 billion As, Ts, Cs, and Gs that make up our DNA. We now think that the difference between being a typical Nigerian with dark skin or a typical Finn with very pale skin comes largely from a change in one pair of letters. That’s one in 3.2 billion pairs. It’s hard to believe a change that small creates such a large effect, but here’s how it seems to work.
Drawing by Robert Krulwich.
Skin colour depends on the pigment melanin. Melanin is produced by cells called melanocytes. Lots of melanin makes you darker. Less melanin makes you lighter. When David Kingsley, a Stanford professor of developmental biology, and his team looked at the billions of letters that make up our DNA.
They found buried in this alphabet soup a bunch of letters that make up a gene called Kitlg (for KIT ligand, a protein-coding gene). For purposes of illustration (and this is totally fanciful) I’m imagining the Kitlg gene as some letters way up near the right corner of our clump of DNA. What does Kitlg do? Well, it’s found in many animals. Fish have it. We have it. It seems to help make blood, sperm, and cells that produce colour, especially melanin.
Here comes the surprising part. If you look at Kitlg closely, writes science journalist Kat Arney, it seems to be a “regulatory” gene—it tells other genes what to do. While some genes are construction workers, building blood cells, muscle cells, and liver cells, Kitlg is a foreman. It tells the bone guys, the blood guys, the liver guys, “Stop!” or “Go!” Instead of a builder, Kitlg seems to be a turn-on-er or a turn-off-er and is, writes Arney, “chock-full of control switches.”
So what does Kitlg switch on? Looking more closely, Kingsley found that Kitlg sends a message to a gene that isn’t exactly close by. It’s some 300,000 letters away (so I’m going to fantasize and put this gene—a second clump of letters—on the lower right corner of our map).
How the signal finds it way through this dense soup of letters is not well known, but when the message lands, it affects the gene that produces melanin. If the signal strongly says, “Go!”, the melanin will turn fully on and will darken our skin and our hair. If the signal is weaker, if it says, “Go a little,” then the skin and hair will stay lighter, even pale.
Finding the Needle in the Haystack
So why do white Europeans consistently have less melanin than West Africans? Kingsley’s team looked at the As, Gs, Cs, and Ts in the switch in both groups and found a single difference.
An “A” in the Africans had become a “G” in the Europeans. A one-letter change. That’s all. But that difference changed melanin production. The “A” was apparently triggering a full “Go!” command. The “G,” on the other hand, wasn’t quite as effective at activating the gene—its signal produced less melanin.
After working with skin cells in his lab, Kingsley was persuaded that this one-in-3.2-billion difference was creating much of the variation we see in our multicolored world. It’s true, of course, that humans come in 30 or so different shades (colour expert Pantone finds almost a hundred), so, writes Kat Arney:
"The difference in the Kitlg switch only explains part of our skin colour, rather than the whole thing. David suspects that there are probably other similar genes and switches out there that add up to give each person their particular hue".
But Kitlg is central. It not only explains skin colour, but it also determines hair colour. In 2014, Kingsley co-authored a paper about blondes. What, he asked, makes a blonde blonde?
Once again he found a single control switch in blonde Europeans that, again, turned out to be one letter different from typically dark-haired people. And, once again, that single difference made it harder to turn on the melanin, the darkening agent, “enough to significantly cut down the melanin production in hair cells and make them fair.”
The Blonde Difference
“Growing up in the 1980s,” writes Arney, “I would often hear jokes about blondes being stupid – and as a brunette (to my shame) I would often repeat them. I now know better, but many people apparently don’t.”
We’re told over and over that hair colour matters, that blondes “have more fun,” that gentlemen prefer them, that they’re dumber than the rest of us, that colour differences reflect a deeper, more subtle divide. When Kingsley’s study came out in 2014, the popular press went ga-ga with blonde jokes, so he decided to turn his science paper into a teaching moment. In a press release he said:
“It’s clear that this hair colour change is occurring through a regulatory mechanism that operates only in the hair. This isn’t something that also affects other traits, like intelligence or personality. The change that causes blonde hair is, literally, only skin deep”.
So now we know. Which is a humbling lesson when you consider, writes Arney, that:
“Countless numbers have been unfairly judged, oppressed or killed throughout history because of the colour of their skin, yet it boils down to little more than a handful of DNA letters in a few genetic switches. For a species named after our intelligence – Homo sapiens translates as ‘man who knows’ – we really are very stupid at times”.
… or easily distracted by surfaces